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 Twounit cells of MKP viewed close to theb axis | |
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| Names | |
|---|---|
| IUPAC names Potassium dihydrogen phosphate[1] | |
| Other names Potassium phosphate monobasic; Phosphoric acid, monopotassium salt; Potassium biphosphate | |
| Identifiers | |
3D model (JSmol) | |
| ChEMBL | |
| ChemSpider |
|
| ECHA InfoCard | 100.029.012 |
| EC Number |
|
| E number | E340(i)(antioxidants, ...) |
| RTECS number |
|
| UNII | |
| |
| |
| Properties | |
| KH 2PO 4 | |
| Molar mass | 136.086 g/mol |
| Appearance | Colourless crystals or white granular or crystalline powder[2] |
| Odor | Odorless[2] |
| Density | 2.338 g/cm3 |
| Melting point | 252.6 °C (486.7 °F; 525.8 K)[4] |
| Boiling point | 400 °C (752 °F; 673 K) , decomposes |
| 22.6 g/100 mL (20 °C) 83.5 g/100 mL (90 °C) | |
| Solubility | Slightly soluble inethanol |
| Acidity (pKa) | 6.86[3] |
| Basicity (pKb) | 11.9 |
Refractive index (nD) | 1.4864 |
| Structure | |
| Tetragonal[5] | |
| I42d | |
a = 0.744 nm,b = 0.744 nm,c = 0.697 nm | |
| Hazards | |
| GHS labelling: | |
 [6] | |
| Warning[6] | |
| H315,H319[6] | |
| P264,P280,P305+P351+P338,P321,P332+P313,P337+P313[6] | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 3200 mg/kg (rat, oral) |
| Safety data sheet (SDS) | External MSDS |
| Related compounds | |
Othercations | Monosodium phosphate Monoammonium phosphate |
Related compounds | Dipotassium phosphate Tripotassium phosphate |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Monopotassium phosphate (MKP) (also,potassium dihydrogen phosphate,KDP, ormonobasic potassium phosphate) is theinorganic compound with the formula KH2PO4. Together withdipotassium phosphate (K2HPO4.(H2O)x) it is often used as afertilizer,food additive, andbuffering agent. The salt often cocrystallizes with the dipotassium salt as well as withphosphoric acid.[7]
Single crystals areparaelectric at room temperature. At temperatures below −150 °C (−238 °F), they becomeferroelectric.
Monopotassium phosphate can exist in severalpolymorphs. At room temperature it formsparaelectric crystals with tetragonal symmetry. Upon cooling to −150 °C (−238 °F) it transforms to aferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to −50 °C (−58 °F) when hydrogen is replaced by deuterium.[8] Heating to 190 °C (374 °F) changes its structure to monoclinic.[9] When heated further, MKP decomposes, by loss of water, to potassium metaphosphate,KPO
3, at 400 °C (752 °F).
| Symmetry | Space group | № | Pearson symbol | a (nm) | b (nm) | c (nm) | Z | Density (g/cm3) | T (°C, °F, K) |
|---|---|---|---|---|---|---|---|---|---|
| Orthorhombic[8] | Fdd2 | 43 | oF48 | 1.0467 | 1.0533 | 0.6926 | 8 | 2.37 | < −150 °C, −238 °F, 123 K |
| Tetragonal[5] | I42d | 122 | tI24 | 0.744 | 0.744 | 0.697 | 4 | 2.34 | −150 to 190 °C, −238 to 374 °F, 123 to 463 K |
| Monoclinic[9] | P21/c | 14 | mP48 | 0.733 | 1.449 | 0.747 | 8 | 190 to 400 °C, 374 to 752 °F, 463 to 673 K |
Monopotassium phosphate is produced by the action ofphosphoric acid onpotassium carbonate. It can then be crystallized into boules, large crystals by dissolving the KDP in hot water and salt, creating a growth solution, placing a seed crystal in the solution and then cooling the solution, done in a holden-type crystallizer, in what is known as solution growth.[10][11][12]
Fertilizer-grade MKP powder contains the equivalent of 52%P
2O
5 and 34%K
2O, and is labeledNPK 0-52-34. MKP powder is often used as anutrient source in thegreenhouse trade and inhydroponics.
Crystals of MKP are used inoptical modulators and fornon-linear optics such assecond-harmonic generation (SHG).Potassium dideuterium phosphate (KDP), with slightly different properties, is also used in nonlinear frequency conversion of laser light. The replacement of protons with deuterons in the crystal shifts the third overtone of the strong OH molecular stretch to longer wavelengths, moving it mostly out of the range of the fundamental line at approximately 1064 nm ofneodymium-based lasers. Regular KDP has absorbances at this wavelength of approximately 4.7–6.3% per cm of thickness while highly deuterated KDP has absorbances of typically less than 0.8% per cm.
Monopotassium phosphate is also used as an ingredient insports drinks such asGatorade andPowerade.
In medicine, monopotassium phosphate is used for phosphate substitution inhypophosphatemia.[13]
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